1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image on a recording material carried on recording material carrying means, such as a transfer drum. Particularly, the present invention pertains to an image forming apparatus for transferring a visible image formed on an image carrying member onto a recording material carried on the recording material carrying means.
2. Description of the Related Art {In a conventional image forming apparatus which employs the electrophotographic process, a toner image formed on an image carrying member, such as a photosensitive drum, is transferred onto a sheet-like transfer material, such as a sheet of paper, by means of a transfer device.
Particularly, in the full-color electrophotographic apparatus, a transfer drum such as that shown in FIGS. 9 and 10 is used as the transfer device for transferring a plurality of toner images on the same transfer material in such a manner that the subsequent toner image is placed on top of the previous one. This transfer drum includes a retaining frame formed by coupling two ring members 5a by a coupling member 5c, as shown in FIG. 9, and a transfer material carrying member which is a recording material carrying sheet, i.e., a flexible transfer material carrying sheet 52, whose four sides, i.e., the hatched portion, are adhered to and thereby wound around the retaining frame, as shown in FIG. 10. A transfer material P is electrostatically attached to the transfer material carrying sheet 52.
However, the aforementioned type of transfer drum which electrostatically attaches the transfer material P has drawbacks in that (1) it requires a corona charger or a charging brush as charge application means and is therefore expensive and (2) attachment of the transfer material P is instable depending on the temperature or humidity of the atmosphere. Hence, a transfer drum shown in FIG. 11 has been proposed. This transfer drum has grippers 51 for mechanically gripping the forward end of the transfer material P so as to allow the transfer material P to be reliably gripped and conveyed.
FIG. 12 illustrates the dimensional relationship between the transfer material carrying sheet 52 and the transfer material P in the aforementioned types of conventional transfer drums. As shown in FIG. 12, the transfer drum has a size which ensures that the transfer material P having the maximum size A3 can be held in a longitudinally fed state (in a state where the longitudinaI direction of the transfer material is substantially identical with the direction in which the transfer drum is moved). Furthermore, the image transferred onto the transfer material is set such that transfer of that image leaves a blank frame (a non-image-forming area with respect to image data), shown in FIG. 12, on the sheet of paper which is the transfer material.
The function of the transfer drum will be explained below with reference to FIG. 13 which illustrates the fullcolor electrophotographic copying machine.
In the copying machine shown in FIG. 13, around a photosensitive drum 1 are disposed a corona charger 2, an exposure optical system 3, a developing device 4, a transfer drum 5, and a cleaner 6. The optical system 3 includes an original scanning portion 3a and a color decomposing filter 3b. The developing device 4 has developers of four colors, that is, a yellow developer 4Y, magenta developer 4M, a cyan developer 4C and a black developer 4K. These four developers are mounted on a housing 4a which can be rotated about a central axis 4b so as to allow development of the four colors to be conducted in sequence.
The transfer drum 5 has ring members 5a, a transfer charger 5b, transfer material grippers 51, an inner charger 5d, and an outer charger 5e. As shown in FIG. 10, a transfer material carrying sheet 52 extends over an open area formed by the ring members 5a and the coupling member 5c, which serve as the frame. The transfer material P is supported and conveyed by this transfer material carrying sheet 52.
To form a full-color image in the copying machine shown in FIG. 13, a green-color-decomposed electrostatic latent image is first formed on the photosensitive drum 1, and the formed latent image is developed by the magenta developer 4M. In the meantime, the transfer material P fed from a transfer material cassette 7 to the transfer drum 5, by means of a conveying system, is gripped by the grippers 51 of the transfer drum 5. As the rotation of the transfer drum 5 proceeds, the toner image on the photosensitive drum 1 is transferred onto the transfer material P by means of the transfer charger 5b. Concurrently with this, the transfer material P is attached to the transfer material carrying sheet 52.
A similar process is repeated for the cyan, 4C, yellow, 4Y, and black 4K, developers to transfer toner images of the four colors. Thereafter, the transfer material P is separated from the transfer drum 5, and the toner on the transfer material is melted and mixed with each other by means of a heat roller fixer 9. The fixed transfer material P is discharged to a tray 10.
However, in the aforementioned conventional transfer device, when the photosensitive drum 1 and the transfer material carrying sheet 52 are located closest to each other, toner particles or dust floating in the image forming apparatus stays at an end area A of the transfer material carrying sheet 52 as shown in FIG. 14(A), in the direction substantially perpendicular to the direction in which the transfer drum is moved. Such toner particles or dust will damage and twist the end portion of the transfer material carrying sheet 52, as shown in FIG. 14 (B) when the copying machine is continuously used. This damage or twisting occurs because the rear surface of the area A is adhered to the ring member 5a, as shown in FIG. 14(A), and the pressure of the photosensitive drum 1 increases in that area, causing the toner particles and dust to be readily fixed to the area A of the transfer material carrying sheet 52.
When the copying machine is operated in the above-described state, adhesion of the toner image located on the photosensitive drum 1, to the transfer material P on the transfer material carrying sheet 52, greatly deteriorates, thus forming a low quality image having a transfer failure at the end portions of the transfer material P, as shown in FIG. 15.